1. Field of the Invention
This invention relates to the field of protective coatings for use on nickel and cobalt base alloys, particularly at high temperatures, to reduce oxidation corrosion.
2. Description of the Prior Art
Nickel and cobalt base superalloys are widely used under conditions of high temperature where oxidation/corrosion are serious problems. Such alloys find particular use in the field of gas turbine engines, where increased efficiency can be obtained by operation at higher temperatures. Under such increased temperatures oxidation/corrosion becomes a greater problem and for this reason current gas turbine engine practice is to use protective coatings on a majority of nickel and cobalt alloy parts which are used at elevated temperatures. The term "oxidation/corrosion" is meant to refer to high temperature interactions between the superalloy or coated superalloy and the environment. The major active element is oxygen, however corrosive effects can result from other elements such as sodium, sulfur and vanadium. The most successful known type of coatings are those which rely on the formation of a continuous layer comprised predominately of aluminum oxide (Al.sub.2 O.sub.3) on the surface of the coating which acts as a diffusion barrier to minimize further reactions. Alumina has been found to be the most effective protective material with regard to oxygen and is also beneficial with regard to most of the other reactive environmental elements. The function of protective coatings is to form a barrier which minimizes the reaction of the environment with the superalloy base material. A major problem encountered with such coatings is that the coefficient of thermal expansion of the alumina layer differs from the coefficient of expansion of the base material and the coating material which are generally similar. During thermal cycling stresses develop between the alumina layer and the coating material. The alumina layer, which is relatively brittle, tends to crack and spall off thus exposing a fresh surface to the deleterious atmosphere. This repeated formation and spallation of the oxide layer causes the reduction of the coating material in aluminum content. When the aluminum level of the coating material drops below a certain point the coating becomes ineffective as an alumina former and the protective benefits of the coating material are lost.
It has been found in the past that the addition of yttrium to the coating material improves the adherence of the alumina layer to the surface of the coating material. Alumina forming coating materials containing yttrium are described in U.S. Pat. Nos. 3,528,861, 3,542,530 3,649,225 and 3,676,085 all of which are assigned to the assignee of the present invention.
Several prior art patents contain reference to the possible use of hafnium in coatings. U.S. Pat. No. 3,025,182 is directed to coatings which are applied by flame spraying and discloses a process in which a mixture of powders of different compositions are flame sprayed onto the surface to be protected. Hafnium is mentioned in passing as a possible component of one of the powders. If the hafnium were to be present it would be present in boride form with the coating composition as applied containing at least 2 percent boron. The emphasis of the patent is on the use of boron as a reducing agent to eliminate the oxide film formed during flame spraying so that the powder particles which are flame sprayed may bond together adequately. U.S. Pat. Nos. 3,535,146 and 3,620,809 disclose a coating process which involves surface alloying of a wide variety of elements onto the surface to be protected. The essence of the invention is the use of a barrier layer between the surface and the coating layer to retard the diffusion of the coating layer into the substrate thereby prolonging the effectiveness of the coating layer. Hafnium is disclosed as one of a wide variety of elements which may be surface alloyed as a protective coating. Neither aluminum, chromium, nor hafnium are required in the processes disclosed in these patents, thus they do not rely on alumina as a protective layer. U.S. Pat. No. 3,547,681 discloses a multilayer coating for use with tantalum substrates. The coating comprises a porous undercoat and an overcoat which is bonded to the undercoat. Hafnium is used in powdered boride form as the porous undercoat. Aluminum is optional and it is therefore evident that the coating does not rely on the formation of an alumina film for surface protection. U.S. Pat. No. 3,746,279 discloses a multilayer protective coating containing a large portion of manganese. In Table IV a coating composition containing hafnium is shown to be inferior to all other coating combinations tested. The coating described in this patent does not rely on alumina as a protective layer.